3 ms·

x86 was always all about backwards compatibility. It's the whole reason why it's such an ugly agglomeration of weird features. lucky for us the clean sheet ia

by rrmm 6y ago

x86 was always all about backwards compatibility. It's the whole reason why it's such an ugly agglomeration of weird features. lucky for us the clean sheet ia432...err i960, er itanium will save us all!

chrisseaton 6y ago
> x86 was always all about backwards compatibility. This is backwards compatible in terms of functionality. It doesn't not work, you just may not get the performance you hope for from taking the instruction set at face value. The same as you won't with memory access due to the cache hierarchy. The performance characteristics of Intel's chips has always been changing. > lucky for us the clean sheet ia432...err i960, er itanium will save us all! These are all dead architectures.
- rrmm 6y ago
  Agreed. I was looking at the quoted article's comment about mmx and x87 being obsolete. No matter how obsolete or old you still have to find room on the die to mush in all this old cruft. Maybe eventually, it'll get pushed into some software emulation on top of more recent extensions.
- reitzensteinm 6y ago
  I don't know who is downvoting you, but you are 100% correct that the behaviour is backwards compatible. Intel chips have here and there reverted operations to microcode, but a Skylake era chip executing MMX instructions in microcode is going to absolutely blow a contemporary chip out of the water. MMX was introduced in Intel processors before out of order execution. It would surprise me if real world performance of any program had regressed even since Core 2.
  - BeeOnRope 6y ago
    MMX performance actually regressed somewhat between Haswell and Skylake, and Skylake is when the sharing described in this post was introduced and there might be a connection there.
    - reitzensteinm 6y ago
      Right, but I'm saying that the regression in performance doesn't cause a backwards compatibility issue. You've written some MMX code twenty years ago on a P5 Pentium? It's still going to run 50x faster on a modern CPU.

by rrmm 6y ago

chrisseaton 6y ago

> x86 was always all about backwards compatibility. This is backwards compatible in terms of functionality. It doesn't not work, you just may not get the performance you hope for from taking the instruction set at face value. The same as you won't with memory access due to the cache hierarchy. The performance characteristics of Intel's chips has always been changing. > lucky for us the clean sheet ia432...err i960, er itanium will save us all! These are all dead architectures.

rrmm 6y ago
Agreed. I was looking at the quoted article's comment about mmx and x87 being obsolete. No matter how obsolete or old you still have to find room on the die to mush in all this old cruft. Maybe eventually, it'll get pushed into some software emulation on top of more recent extensions.
reitzensteinm 6y ago
I don't know who is downvoting you, but you are 100% correct that the behaviour is backwards compatible. Intel chips have here and there reverted operations to microcode, but a Skylake era chip executing MMX instructions in microcode is going to absolutely blow a contemporary chip out of the water. MMX was introduced in Intel processors before out of order execution. It would surprise me if real world performance of any program had regressed even since Core 2.
- BeeOnRope 6y ago
  MMX performance actually regressed somewhat between Haswell and Skylake, and Skylake is when the sharing described in this post was introduced and there might be a connection there.
  - reitzensteinm 6y ago
    Right, but I'm saying that the regression in performance doesn't cause a backwards compatibility issue. You've written some MMX code twenty years ago on a P5 Pentium? It's still going to run 50x faster on a modern CPU.